This invention relates to an optical recording medium and a technique for reproducing the optical recording medium, and particularly to an optical recording medium suitable for use in increasing the recording density, and a method and apparatus for reproducing the same.
This invention is particularly suited for the use of a write once type or erasable optical disk for recording by utilizing the local change of characteristics, for example, change of reflection factor, transmissitivity, depth or the like of the medium due to the irradiation of a light spot on the medium.
In a conventional recording and reproducing scheme for, for example, a filing optical disk, a groove of a constant depth is provided in a spiral form on the disk so that a plurality of grooves are arranged to have an equal spacing in the radial direction of the disk, and a light spot is irradiated on the groove or the region between the grooves so that information is recorded, reproduced or erased along the center thereof. This groove serves as a guide for guiding the light spot along the track. In order to reduce the noise upon recording and reproduction, the width of the region between the grooves (or the distance between the track) is desired to be smaller than the diameter of each of the recorded pits which are recorded as information on the disk. Such a high density recording as to record the tracks of which the spacing is smaller than the diameter of the light beam spot has encountered with the following two problems.
The first problem is the problem with the tracking. In general, the positional shift of the light spot in the radial direction with respect to the center of the track, or the shift of the light spot out of track is detected by the so-called "push-pull system" which is well known. In other words, this system utilizes the action of the structure of grooves peripodically arranged in the radial direction as a diffraction grating to the light spot. If the distance between the tracks, or the distance between the grooves is reduced in order to increase the storage capacity per unit area, or the storage density, the spatial frequency in the radial direction of the disk which frequency is expressed by the reciprocal of the period of the grooves becomes close to the cutoff frequency (expressed by the reciprocal of the light spot diameter) of a reading optical system, and thus it is difficult to detect the positional shift of the light spot in the radial direction. This problem is the first problem.
The second problem is crosstalk that undesired information is leaked from the adjacent tracks. When the distance between the tracks is smaller than the light spot diameter, the light spot crosses part of the adjacent tracks so that undesired information is mixed into the necessary information of the target track even if the light spot just traces the center of the target track. In other words, the crosstalk increases.
To cope with these problems in the prior art, the JA-A-57-105828 proposes the following method. As shown in FIG. 3A, the disk surface is of "V-letter shape" or "inverted trapezoidal shape" in cross-section, and the light spot is moved along the slope of each groove to record thereon or reproduce therefrom information pits. In this method, the period of the grooves is the same as the distance between the grooves in the prior art, but the distance between the tracks in 1/2 that in the prior art. Thus, the tracking error can be detected, or the first problem can be solved. Moreover, since the information track surface is a slope, the distribution of the diffracted light is controlled by the control of the shape of the slope, so that only the diffracted light from the target information track 11 is distributed on the outer periphery of the exit pupil, with the effect of the cross-talk from the adjacent information track 12 or 13 being reduced, thus solving the second problem.
In addition, the JA-A-61-192047 proposed the following method. As shown in FIG. 3B, raised tracks 14 and recessed tracks 15 are alternately formed on the disk in a spiral shape or concentrically in the radial direction, and also a light guiding groove 16 is provided on each track. The optical guide groove 16 is smaller in width than the diameter of the recorded pits 17 and has (1/4 to 1/8) .lambda. in depth where .lambda. is the wavelength of the laser light. The recording density on the disk of this structure is increased by the recording and reproduction on both raised tracks 14 and recessed tracks 15. Moreover, the light guiding grooves 16 on the tracks can be used for high-precision tracking control, thus reducing the cross-talk.
In addition, the JP-A-54-136303 proposes another method wherein, as shown in FIG. 3C, information tracks 20 to 23 of pits 18 and 19 which are .lambda./4 and .lambda./8 deep, respectively are arranged on the disk surface 3 in the radial direction. The distribution of the diffracted light is controlled by making the depths of the adjacent information pits different. That is, upon reproduction of the information tracks 20, 22 of .lambda./8 in depth, the difference between the signals from the light receiving surfaces of the two-divided photodetector is detected as a reproduced signal, and upon reproduction of the information tracks 21, 23 .lambda./4 deep, the sum of the signals therefrom is detected as the reproduced signal, thus reducing the cross-talk.
In the proposed method in the JP-A-57-105828 (FIG. 3A), the master disk having grooves of the "V-shape" or "inverted trapezoidal shape" in cross-section is produced by mechanical cutting of a metal plate with a diamond stylus of which the tip has a "V-shape" or "trapezoidal shape". In order to polish the sloped surfaces of the grooves on which the recorded pits are to be formed, up to a high-precision mirror-like finish, it is necessary to use the usual cutting system for fine patterns, or a laser cutting process in which laser light is irradiated on the surface that is coated with a resist sensitive to an ultraviolet ray (a photoresist), and the exposed areas are developed to form grooves. However, it is difficult to produces the above master disk by use of this laser cutting process.
In the proposed method in the JP-A-61-192047 (FIG. 3B), to polish the raised and recessed surfaces of grooves on which pits are to be recorded, the mechanical cutting process must be used as in the above prior art. Moreover, when the light guiding groove 16 is formed in each track, the reading of the information pits is greatly affected by noise from the light guiding grooves.
In the JP-A-54-13603 (FIG. 3C), the information pits on the adjacent tracks must have different depths. This is possible in such media as "read-only optical disk" in which information pits are formed in advance by a high-precision process control, for example, the laser cutting apparatus or the like. However, in such media as "optical disks for data file" in which information pits are recorded, reproduced and erased by a simple apparatus on the user's side, it is practically difficult to precisely record pits of different target depths on every track.